1. Field of the Invention
The present invention relates to a method of controlling a fusing temperature of an electrophotographic imaging apparatus, such as a printer, a copy machine, or a facsimile, and more particularly, to a method of controlling a fusing temperature of a fusing apparatus having a rubber layer thereon.
2. Description of the Related Art
Electrophotographic imaging apparatuses include a fusing apparatus that heats a sheet of paper to which a toner image is transferred, to instantaneously fuse and fix the toner image on the paper. The fusing apparatus includes a fusing roller that is heated to fuse the toner image on the paper, and a pressing roller that pushes the paper against the fusing roller to tightly support the fusing roller while the paper is fed therebetween.
FIG. 1 is a cross-sectional view of a conventional fusing roller 10 in which a halogen lamp (heater) 12 is installed as a heating source. FIG. 2 is a cross-sectional view of a fusing apparatus provided with the fusing roller 10 of FIG. 1.
Referring to FIG. 1, the fusing roller 10 includes a cylindrical roller 11 and the halogen lamp 12 installed at a core of the roller 11. A toner-releasing coating layer 11a made of Teflon is formed on a surface of the roller 11. The halogen lamp 12 generates heat to heat the fusing roller 10.
Referring to FIG. 2, a pressing roller 13 is positioned below the fusing roller 10, and a sheet of paper 14 is fed between the pressing roller 13 and the fusing roller 10. The pressing roller 13 is elastically supported by a spring 13a to contact the fusing roller 10 and to apply a predetermined pressure to push the paper 14 toward the fusing roller 11. While the paper 14 to which an unstable toner image has been transferred passes between the fusing roller 10 and the pressing roller 13, the toner image formed of toner particles 14a is fused onto the paper 14 by pressure and heat.
A thermistor 15 that measures a surface (fusing) temperature of the fusing roller 11, and a thermostat 16 that cuts off a power supply to the halogen lamp 12 from an external power source when the surface temperature of the fusing roller 10 exceeds a predetermined set value, are installed adjacent to the fusing roller 10. The thermistor 15 measures the surface temperature of the fusing roller 10 to transmit an electrical signal corresponding to the measured surface temperature to a controller (not shown) of a printer (not shown). The controller controls the external power source to supply the power supply to the halogen lamp 12 based on the measured temperature to keep the surface temperature of the fusing roller 10 within a given range. When the measured temperature of the fusing roller 10 exceeds the predetermined set value as a result of failure in a temperature control by the thermistor 15 and the controller, a contact (not shown) of the thermostat 16 becomes open to cut off the power supply supplied to the halogen lamp 12 from the external power source.
In the above-described fusing roller 10 having the halogen lamp 12 as the heating source, only the toner-releasing coating layer 11a having a thickness of 20-30 μm is formed on the cylindrical roller 11. Accordingly, there is rarely a difference in surface temperatures between the roller 11 and the toner-releasing coating layer 11a, so that the surface temperature of the fusing roller 10 can be measured from the toner image-releasing coating layer 11a to control the external power source or the halogen lamp 12 to supply the power supply to the halogen lamp 12 by an on-off control.
FIG. 3 is a flowchart showing the on-off control of the fusing apparatus in the electrophotographic imaging apparatus. Referring to FIGS. 2 and 3, the surface temperature of the fusing roller 10 is measured at a predetermined interval, for example, at 100 ms in operation 40.
The measured temperature of the fusing roller 10 is compared with a target fusing temperature in operation 42. If the measured temperature of the fusing roller 10 is lower than the target fusing temperature, the halogen lamp 12 is turned onin operation 44. If the measured temperature of the fusing roller 10 is higher than or equal to the target fusing temperature, the halogen lamp 12 is turned off in operation 46. After operation 44 or 46, operation 40 of measuring the surface temperature of the fusing roller 10 is repeated. In other words, a temperature of the fusing roller 10 can be simply controlled to be constant by measuring the surface temperature at intervals and controlling the halogen lamp 12 or the external power source to supply the power supply to the halogen lamp 12 by the on-off control.
However, the fusing apparatus used in a high-speed printer capable of printing 25 sheets of paper a minute or in a color printer requires a greater fusing nip between the fusing roller 10 and the pressing roller 13 to obtain a longer fusing duration and a higher fusing efficiency. To this end, a method of disposing a rubber layer having a predetermined thickness between the toner-releasing coating layer 11a and the cylindrical roller 11 of the fusing roller 10 has been suggested.
Referring to FIG. 4, a fusing roller 50 includes a cylindrical roller 51 and a halogen lamp 52 installed at the core of the cylindrical roller 51. The cylindrical roller 51 is formed of aluminum with a thickness of 1.5 mm, a rubber layer 53 having a thickness of 1.5 mm is formed on the cylindrical roller 51, and a Teflon coating layer 53a having a thickness of 20-30 μm is formed on the rubber layer 53. The halogen lamp 52 generates the heat in the cylindrical roller 51, and the cylindrical roller 51 is heated by the heat radiated from the halogen lamp 52 and transfers the heat to the rubber layer 53 and the coating layer 53a. 
FIG. 5 is a graph of temperature profiles with respect to time at various positions in a radial direction of the fusing roller 50 when a predetermined power is supplied to the halogen lamp 52 of the fusing roller 50 of FIG. 4. Referring to FIG. 5, in the cylindrical roller 51 having a thickness ranging from a radial distance of 13 mm, which is measured from a core (center) of the fusing roller 50 in a radial direction, to 14.5 mm, i.e., an outer circumference of the cylindrical roller 51, the temperature of the cylindrical roller 51 is constant throughout its thickness when being heated and measured because of a high thermal conductivity of the cylindrical roller 51 made of aluminum.
In the rubber layer 53 having a thickness ranging from the radial distance of 14.5 mm to 16 mm, the measured temperature of the rubber layer 53 tends to drop greatly with the increase of the radial distance from the core of the cylindrical roller 51. This is because the thermal conductivity of the rubber layer 53 is so low that a heat transfer rate (speed) from the cylindrical roller 51 to a surface of the rubber layer 53 is very slow. For example, for a heating duration of 90 seconds, the temperature of the cylindrical roller 51 reaches 230° C. due to a thickness of the rubber layer 53 while the surface temperature of the fusing roller 50 is as low as 180° C.
When a temperature control to the fusing roller 50 having the thick rubber layer 53 is performed by a general on-off control method, the following problems occur. When the surface temperature of the fusing roller 50 reaches the target temperature, for example, 180° C., the temperature of the cylindrical roller 51 is 230° C. If the halogen lamp 52 is turned off at this time, the temperature of the cylindrical roller 51 immediately drops while the surface temperature of the rubber layer 53 continues to rise when the cylindrical roller 51 has a higher temperature than that of the rubber layer 53. As a result, the surface temperature of the fusing roller 50 rises above the target fusing temperature.
When the surface temperature of the fusing roller 50 is lower than the target fusing temperature in a print mode, the halogen lamp 52 is turned on to heat the fusing roller 50. At this time, if the surface temperature of the fusing roller 50 is maintained below the target fusing temperature during the printing mode, the temperature of the cylindrical roller 51 may increase to a certain temperature—higher than the target temperature. As a result, the rubber layer 53 may be thermally deformed.
Furthermore, since a power control period is too short according to this method, the halogen lamp 52 must be turned on and off frequently, thereby causing flicker problems.